Carrier control, most likely. I'm sure HTC are being lobbied hard by carriers who want to be able to use artificial restrictions to segment their offerings.
Exactly. Phone manufactures are in the pocket of the carriers (as google proved with the Nexus one, you can't sell phones without carrier relations) and carriers don't like rooting as it limited their options to block certain software and services (e.g. VOIP / tethering).
Perhaps it's setting things up for the future, but the carrier in question here, T Mobile, is notable for its openness in not blocking tethering or voip.
I should imagine the decision about how much protection is applied to the system is made long before the device is accepted on various carriers. Ok, this is a 'made for T-mobile' device, but its likely it will appear in almost the same form on other carriers under another name and HTC branding. So, it would be easier to build in the protection other networks want at this this early stage.
It needn't be T-Mobile to have prodded HTC in this direction. In fact, it's more likely to have been another carrier who said "We'll only start to carry this phone if you lock it down like this."
